Dental CAD/CAM system for obtaining a position match of 3D data sets

ABSTRACT

Disclosed is a method for designing tooth surfaces of a digital dental prosthetic item existing as a 3D data set using a first 3D model of a preparation site and/or of a dental prosthetic item and a second 3D model, which second model comprises regions which match some regions on the first 3D model and regions which differ from other regions of the first 3D model, the non-matching regions containing some of the surface information required for the dental prosthetic item, wherein at least three pairs (P 1 , P 2 , P 3 ) of points (P 11 , P 12 ; P 21 , P 22 ; P 31 , P 32 ) corresponding to each other are selected on the matching region on the first 3D model (A) and the second 3D model (A′), that the positional correlation of the second 3D model with reference to the first 3D model is determined with reference to the at least three pairs (P 1 , P 2 , P 3 ), and that portions of the non-matching regions of the first and second 3D models are implemented for designing the tooth surface of the dental prosthetic item taking into consideration the positional correlation of these models relative to each other.

This application is a division of U.S. application Ser. No. 11/368,535,filed Mar. 7, 2006, which claimed the priority of German Application No.10 2005 011 066.5, filed Mar. 8, 2005, and is related to U.S.Provisional Application No. 60/749,612, filed Dec. 13, 2005, thepriority of which is hereby claimed. The contents of U.S. applicationSer. No. 11/368,535 are hereby incorporated by reference herein in theirentireties, as if set forth fully herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to part of a process for the manufacture ofdental prosthetic items in a dental CAD system. In the dental CAD/CAMsystem “CEREC” (registered trademark of Sirona Dental Systems GmbH,Germany), the operator performs a 3D scan of a prepared tooth and itsadjacent teeth, from which scan a data representation of the 3D model ofthe region scanned is produced in the memory unit of a computer anddisplayed.

2. Description of Related Art

In order to copy, say, the chewing surface of a tooth prior topreparation, the tooth can be scanned prior to preparation, togetherwith the adjacent teeth, and a corresponding 3D model can then becomputed.

Another variant comprises the scanning of an impression of the oppositejaw, in order to achieve an optimum fit of the dental prosthetic item onsaid opposite jaw.

In order to be able to use the information of the unprepared tooth orthe impression of the opposite jaw, it is necessary to achieve asufficiently exact geometric alignment of the corresponding models tothe model of the prepared tooth. The information that makes suchalignment possible must therefore be contained in the models.

The use of the surface of the adjacent teeth, with no change in thesurface between the scans, is known in the prior art. This normallytakes place automatically via the software, by means of which regions ofthe 3D data set corresponding to each other are sought and found.

There are cases in which this automatic process fails, due, for example,to the poor quality of the scans, too few unaltered parts of the 3Dmodels, or interfering image components such as cofferdams or rolls ofcellulose wadding.

Despite this, and in order to be able to use this information eitherfrom the scans prior to preparation or from the opposite jaw, theposition match of the models must be obtained in a different way.

OBJECTS AND SUMMARY OF THE INVENTION

The invention relates to a method for designing dental surfaces for adigital dental prosthetic item existing as a 3D data set by means of afirst 3D model of a preparation site and/or of a yet to be milled dentalprosthetic item and a second 3D model, which second model includesregions which match some regions on the first 3D model and regions whichdiffer from other regions of said first 3D model.

The non-matching regions contain some of the surface informationrequired for the dental prosthetic item.

At least three pairs of points corresponding to each other are selectedon the matching region on the first 3D model A and the second 3D modelA′, and the positional correlation of said second 3D model (A′) relativeto said first 3D model (A) is determined with reference to the at leastthree pairs of points.

The portions of the non-matching regions of the first 3D model (A) andof the second 3D model (A′) are implemented for designing the toothsurface of the dental prosthetic item taking into consideration thepositional correlation of these 3D models relative to each other.

The 3D model (A) can, for example, comprise a digital representation ofa prepared tooth with its adjacent teeth and the 3D model (A′) a digitalrepresentation in approximately the same preparation site of animpression of the opposite jaw. The matching regions would then compriseat least portions of the occlusal surfaces of the adjacent teeth of thefirst 3D model (A) and the opposite occlusal surfaces of the opposingteeth in the opposite jaw of the second 3D model (A′). The non-matchingregions would then comprise at least portions of the surface of theprepared tooth in the first 3D model (A) and the occlusal surface of theopposing tooth in the second 3D model (A′) and could be implemented fordesigning the tooth surface of the dental prosthetic item for theprepared tooth.

The alignment of points and regions on the second 3D model (A′) and thefirst 3D model (A) by way of the positional correlation is known in theprior art.

The alignment of the points is achieved by intervention by the operator.

In order to achieve unambiguous determination of the positionalcorrelation of the first 3D model (A) relative to the second 3D model(A′), at least three pairs of points in the matching regions must beselected by the operator, and these three pairs of points may not be inline.

Furthermore, the invention relates to a method for designing dentalsurfaces of a digital dental prosthetic item existing as a 3D data setby means of a first 3D model (A) of a preparation site and/or a dentalprosthetic item yet to be milled and a second 3D model (A′), whichsecond model (A′) has regions matching some regions of the first 3Dmodel (A) and regions which do not match other regions of the first 3Dmodel (A), which non-matching regions contain some of the surfaceinformation required for the dental prosthetic item. A plurality ofpoints forming a region of the surface is selected from the matchingregion on the first 3D model (A) and the second 3D model (A′), and thepositional correlation of said second 3D model (A′) relative to thefirst 3D model (A) is determined with reference to the plurality ofpoints in said region. The portions of the non-matching regions of thefirst 3D model (A) and of the second 3D model (A′) are used fordesigning the tooth surface of the dental prosthetic item taking intoconsideration the positional correlation of these 3D models relative toeach other.

Advantageously, a plurality of regions having a plurality of points canbe determined.

In a development of the invention, the sum of the distances between theestablished points on the first 3D model (A) and the established pointson the second 3D model (A′) or between the points in the matchingregions is minimized for the determination of the positionalcorrelation.

When the points are selected by the operator small deviations from theactually matching points may occur. The sum of the distances of theselected pairs of points is minimized in order to keep this error down.Assuming that the error occurring when the points are selected by theoperator is approximately constant, the actual positional correlationwill be determine more accurately as the number of selected pairs ofpoints is increased.

Furthermore, the invention embraces a method for designing dentalsurfaces of a digital dental prosthetic item existing as a 3D data setby means of a first 3D model of a preparation site and/or of a dentalprosthetic item yet to be milled and a second 3D model, which secondmodel has regions matching some regions on the first 3D model and otherregions which do not match other regions on the first 3D model. Thenon-matching regions contain some of the surface information requiredfor the dental prosthetic item. A pair of points corresponding to eachother is selected on the matching region on said first 3D model and saidsecond 3D model, and the second 3D model is moved in the display so asto coincide with the first 3D model at this point. The second 3D model(A′) can then be adjusted, via input means, relatively to the first 3Dmodel (A) about the at least one point, and a positional correlation isdetermined with reference to an adjusted position. The portions of thenon-matching regions of the first and second 3D models are thenimplemented for designing the tooth surface of the dental prostheticitem taking into consideration the positional correlation of said 3Dmodels relative to one another.

The selection of the point and the adjustment are performed by theoperator.

The second 3D model is then manipulated around the selected coincidencepoint until the first 3D model matches the second 3D model at as manypoints as possible. The total adjustment can then be implemented todetermine the positional correlation.

According to the invention, a first transformation is performed withreference to the established positional correlation.

It is possible to ascertain the first transformation from the positionalcorrelation, and when the first transformation has been carried out, thesecond 3D model can be moved to coincide with the first 3D model.

In a particularly advantageous development, those regions which, afterexecution of the first transformation, show a smaller difference inheight than a specified maximum value between the first 3D model and thesecond 3D model are used as the basis for finding an additionalpositional correlation.

That is to say, those regions which show a greater difference in heightthan the maximum value are not implemented for determination of anadditional positional correlation. In this way, pairs of pointsinaccurately selected by the operator are screened out.

A second transformation is advantageously performed with reference tosaid additional positional correlation.

After execution of the first transformation, a second transformation isthus carried out in order to make the second 3D model coincide with thefirst 3D model even more accurately.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are illustrated in the drawings,in which:

FIG. 1 shows two 3D models A and A′;

FIG. 2 shows the positional correlation illustrated in abstract form bya first coordinate system K1 and a second coordinate system K2;

FIG. 3 a illustrates another method for obtaining a positionalcorrelation by means of a first step;

FIG. 3 b illustrates the rotation of the coordinate system K2′ of FIG. 3a;

FIG. 4 illustrates a first optimization for determination of thepositional correlation;

FIG. 5 illustrates another optimization for determination of thepositional correlation, and

FIG. 6 illustrates an example of a dental prosthetic item, such as adigital dental prosthetic item.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT OF THE INVENTION

Two 3D models A and N are shown in FIG. 1. The 3D model A is a digitalrepresentation of a prepared tooth 1 with its adjacent teeth 2, 3. The3D model N is a data representation in approximately the samepreparation site, wherein, however, an impression 5 of the opposite jawis contained in an impression compound 4 and wherein the adjacent teeth2′, 3′ are predominantly covered by the impression compound 4. Parts ofthe impression 5 should be taken into account when designing a dentalprosthetic item (such as, e.g., item 6 of FIG. 6), in this case thechewing surface in said impression 5. Up to this point, the two 3Dmodels A, A′ do not yet have any spatial relationship to each other. Bycomparing distinctive surface points or regions, one can see with thenaked eye that there are surface points or regions on the surface thatcorrespond to each other in the two models. In particular, the surfacesare edges or crests.

Both of the 3D models A, A′ are represented at the same degree ofmagnification and hence match in terms of their displayed dimensions. Inprinciple, representation of the models on a different scale is alsopossible, because the regions represented are always based on absolutedata.

The operator can therefore define points P11-P32 or regions B11-B32 ofadjacent points, which points or regions reproduce the same object partsin the two 3D models A, A′, in this case parts of the adjacent teeth 2,2′; 3, 3′.

By marking point pairs P11, P12; P21, P22; P31, P32, wherein in eachcase one point P11, P21, P31 of a pair lies on the 3D model A and theother point P12, P22, P32 of the pair lies on the 3D model A′, a clearpositional correlation can be obtained using mathematical methods knownin the prior art. This is likewise possible when the representationsdiffer in size.

In FIG. 2, the positional correlation is illustrated in abstract form bymeans of a first coordinate system K1 and a second coordinate system K2and a transformation T for giving the correlation position. Point pairs(P11, P12), (P21, P22) (P31, P32) are selected in both coordinatesystems, from which point pairs the transformation T is calculated. Thecoordinate system K2 may be reproduced therewith on the coordinatesystem K1 according to the following mathematical formula:K ₁ =T*K ₂

Another method for obtaining a positional correlation between the twocoordinate systems K1, K2 is illustrated in FIG. 3 a. First of all, afirst transformation T is calculated with reference to a point pair(P01, P02), which transformation reproduces the point P02 over the pointP01 and contains exclusively one translation fraction.

Then a rotation of the coordinate system K2′ about the common point P0through the angles φ1, φ2, φ3 of the axes x, y and z is performed by theoperator, as illustrated in FIG. 3 b, in order to bring aboutcoincidence of the two coordinate systems K1, K2 in the coordinatesystem K1. A transformation R is determined therefrom, whichtransformation contains the angle of rotation only and no translationfraction. Lastly, a positional correlation is calculated for thecoordinate systems K1, K2:K ₁ =R*(T*K ₂)

FIG. 4 illustrates how the positional correlation of the three pointpairs P1, P2, P3 is determined. To this end, the transformation issought in which the three point pairs P1, P2, P3 are disposed relativeto each other such that the spacing between the points (P11, P12′),(P21, P22′) and (P31, P32′) and the points P12′, P22′ and P32′ after thetransformation T of the points P12, P22, and P32 of the 3D model A′ isminimal. This is accomplished with algorithms known per se using thefollowing formula, in which n=3 in the present case:

$\min{\sum\limits_{i = 1}^{n}\;{{P_{il} - {T \cdot P_{i\; 2}}}}}$

As a consequence of the transformation T obtained in this manner, thepoints P12, P22 and P32 are thus displayed over the points P12′, P22′and P32′ and the transformation can also be applied to the rest of the3D model A′ in order to obtain the positional correlation of singlepoints or selected regions.

This applies similarly to a region generated from a plurality of points.

An optimization is illustrated in FIG. 5. Based on the assumption that,after performing the first transformation, image regions with slightchanges between the two models will lie close to each other andtherefore be spaced at short distances, those regions exceedingpredefined maximum spacing will be screened out in order to optimize thetransformation.

As an example of the profile of a dental prosthetic item surface as a 3Dmodel, a curve G2′ is illustrated together with a curve G1 in the x,zplane from the first coordinate system, which curve G2′ was generatedfrom a curve G2 (not shown), from the second coordinate system after thefirst transformation. The profile of the curve G2′ comprises regions I,III, which regions are separated from the curve G1 at intervals within amaximum value c, whereas, by contrast, the deviation is clearly greaterthan the maximum value c in the regions II and IV. Said regions II andIV are considered as defects and are omitted in the calculation of thefinal transformation. The positional correlation is therefore onlycalculated with those values of the regions I and III which lie withinthe maximum limit ε.

Ideally, the curves in regions I and III coincide after this additionaltransformation.

In order to achieve this good match, it is self-explanatory that thepoint pairs for the first transformation should fall within the regionsI and III.

1. A system, comprising: a memory arranged to store a first 3D model ofat least one of a preparation site and a digital dental prosthetic item,and a second 3D model that includes regions which match at least someregions on the first 3D model and non-matching regions which differ fromat least some other regions of said first 3D model, the non-matchingregions containing at least some surface information useable forconstructing the digital dental prosthetic item; and a controller,responsive to at least three pairs of points being selected on at leastone of the matching regions of the first 3D model and the second 3Dmodel to (1) determine a positional correlation of said second 3D modelwith reference to said first 3D model with reference to said at leastthree pairs, and (2) minimize a sum of distances of the points on thefirst 3D model from the points on the second 3D model for the positionalcorrelation, portions of the non-matching regions of said first andsecond 3D models being useable for designing the at least one toothsurface of the digital dental prosthetic item based on the positionalcorrelation.
 2. A system, comprising: a memory arranged to store a first3D model of at least one of a preparation site and a digital dentalprosthetic item, and a second 3D model, the second 3D model includingregions which match at least some regions on the first 3D model andnon-matching regions which differ from at least some other regions ofsaid first 3D model, the non-matching regions containing at least somesurface information useable for constructing the digital dentalprosthetic item; and a controller, responsive to a plurality of pointsbeing selected on at least one of the matching regions of the first 3Dmodel and the second 3D model, to (1) determine a positional correlationof at least one matching region of said second 3D model with referenceto the first 3D model, with reference to said plurality of points, and(2) minimize a sum of distances of regions of the first 3D model fromthe regions of the second 3D model for the positional correlation,portions of the non-matching regions of said first and second 3D modelsuseable for designing at least one tooth surface of the digital dentalprosthetic item based on the positional correlation.
 3. The system asdefined in claim 2, wherein the controller also is operable to determinea plurality of regions having a plurality of points.
 4. A system,comprising: a memory arranged to store a first 3D model of at least oneof a preparation site and a digital dental prosthetic item and a second3D model, the second 3D model including regions which match at leastsome regions on the first 3D model and non-matching regions which differfrom at least some other regions of said first 3D model, thenon-matching regions containing at least some surface informationrequired for the digital dental prosthetic item; a display arranged todisplay the first and second 3D models; and a controller, responsive toselection of a pair of points corresponding to each other on thematching regions on said first 3D model and said second 3D model, by:(a) moving said second 3D model in the display so as to coincide withsaid first 3D model at least one of the points, (b) adjusting saidsecond 3D model about the at least one point relatively to said first 3Dmodel to form an adjusted model, and (c) determining a positionalcorrelation with reference to the adjusted model, based on minimizing asum of a distance between the pair of points, at least some of thenon-matching regions of the first and second 3D models being useable fordesigning at least one tooth surface of the digital dental prostheticitem based on the positional correlation.
 5. The system as defined inclaim 1, wherein the controller performs the positional correlationbased on a result of performing a transformation.
 6. A system,comprising: a memory, arranged to store a first 3D model of at least oneof a preparation site and a digital dental prosthetic item, and a second3D model, the second 3D model including regions which match at leastsome regions on the first 3D model and non-matching regions which differfrom at least some other regions of said first 3D model, thenon-matching regions containing at least some surface informationrequired for the digital dental prosthetic item; and a controllerresponsive to selection of at least three pairs of points correspondingto each other on the first and second 3D models, the at least threepairs being selected on at least one of the matching regions of thefirst 3D model and the second 3D model, to (1) determine a positionalcorrelation of said second 3D model with reference to said first 3Dmodel with reference to said at least three pairs, and (2) determine afirst transformation with reference to at least one of the pairs ofpoints, and (3) performing an additional positional correlation usingthose ones of the regions, which, following execution of the firsttransformation, show a smaller difference in height than a specifiedmaximum value (c) between said first 3D model and said second 3D model,portions of the non-matching regions of said first and second 3D modelsbeing useable for designing at least one tooth surface of the digitaldental prosthetic item based on the positional correlation.
 7. Thesystem as defined in claim 6, wherein the controller also is arranged toperform a second transformation.
 8. The system as defined in claim 1,wherein the preparation site includes at least one dental structure. 9.The system as defined in claim 8, wherein the second 3D model is a modelof a material having an impression of the at least one dental structure.10. The system as defined in claim 2, wherein the preparation siteincludes at least one dental structure.
 11. The system as defined inclaim 10, wherein the second 3D model is a model of a material having animpression of the at least one dental structure.
 12. The system asdefined in claim 4, wherein the preparation site includes at least onedental structure.
 13. The system as defined in claim 12, wherein thesecond 3D model is a model of a material having an impression of the atleast one dental structure.
 14. The system as defined in claim 6,wherein the preparation site includes at least one dental structure. 15.The system as defined in claim 14, wherein the second 3D model is amodel of a material having an impression of the at least one dentalstructure.